NAM, GFS, UKMET, NOGAPS, and Canadian all in. Keep in mind, there will be anywhere from 6 to 11 model runs for each of these models, plus the European, for a total of about 46 computer models for us to look at between now and Wednesday evening. So, let’s not put too much stock in the output now.
The basic idea is that an upper-level disturbance will help form a low pressure area along a front near the Gulf coast Wednesday, which will bring moisture up and over the cold air in place near the ground. Since the air near the ground will be dry at first, some of the precipitation will melt/evaporate as it comes down, cooling the atmosphere to near freezing, allowing some mixture of rain and snow over the northern half of Alabama. The exact position of the low pressure area and the location of the freezing line at 850 mb (about 4,000 feet), the critical level in the atmosphere for determining snow or rain, are very hard to tell this far out. During the day on Thursday, warm air aloft should come in enough to change the precipitation to mainly rain. As Brian Peters points out, there could even be a few spots with light icing, if the surface cold air holds on longer with warm air coming in aloft. We’ll have to see.
The new 00 UTC NAM may be the most aggressive with snow for BHM. It shows the surface low south of Pensacola by Thursday morning, with over-running type precipitation coming in Wednesdsay evening. The NAM shows 2.3″ of snow in BHM between midnight and 5 am, turning over to rain by 6 am. It shows 850 mb temps plenty cold enough for snow (0 C) for about 12 hours over the northern half of Alabama, with temps as cold as -3 C in northwest Alabama.
The GFS keeps the cold air a little farther to the north, confining the best chances for significant wintry precipitation to the Tennessee Valley. But, it does show over 1″ of snow in BHM Wednesday evening, changing to rain by midnight.
The UKMET is only available through 00 UTC (6 pm Wednesday), and it shows the rain/snow line very close to BHM.
The NOGAPS has the rain/snow line from Florence to Anniston.
So, let’s wait and see. This is the best chance for snow we’ve seen this year. It actually looks a bit like the February 1, 2007 event in Huntsville, in which precip started as snow, changed to freezing rain briefly, then rain in the morning.
